For the synthesis process of acetals having the following structure: ##STR5## sufficient investigations have not yet been made and as a general process, there is a process described in Japanese Patent Application No. 3-145175 (corresponding to U.S. patent application Ser. No. 885,359 and EP514896A). Practically, according to the foregoing process as shown in following Scheme (I), by forming a methylol by the reaction of compound (A) and paraformaldehyde, compound (B) is synthesized and by reacting compound (B) and mercaptoazole in the presence of zinc iodide, desired compound (C) is synthesized. ##STR6##
However, the foregoing process has the following problems, that is, i) 2 steps are required for obtaining the desired product, ii) the yield in each step is low, iii) since the yield for compound (B), which is an intermediate product, is low and the crystallinity thereof is inferior, it is very difficult to isolate the compound (B) by means other than a column purification, iv) furthermore, in the synthesis of compound (B), paraformaldehyde deposits onto a cooling pipe, and v) if the purity of compound (B) is not high, the synthetic reaction of compound (C) can not be carried out well, i.e., the yield of compound (C) is low. For example, in the example for synthesizing the compound in EP 514896A cited above the yield is only 9.6%.